For most low-side switches I've seen, an NMOS device is used. I am wondering, is it not possible to use a PMOS device in a low-side switch?

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    \$\begingroup\$ Yes. There must be something more to this to prompt the question though. Would you like to share to make this a bit more worthwhile? \$\endgroup\$ – Heath Raftery Jun 2 '19 at 1:28
  • \$\begingroup\$ @HeathRaftery Nothing really. I was just designing some schematic and though why I don't see any high side switches with PMOS devices, seemed like it would work in my brain. \$\endgroup\$ – AlfroJang80 Jun 2 '19 at 2:21

Yes, it's possible. But just like with a high-side NMOS switch, you'd need to drive the gate beyond the supply rail (below Gnd, in this case) in order to switch it fully on.

While there's some motivation to use high-side NMOS switches — they perform better than the equivalent PMOS devices — there's no equivalent motivation to use a PMOS on the low side.

  • \$\begingroup\$ Curious, why do you need to drive the gate below GND to switch it fully on? I mean once it's below the Vth, isn't that enough? \$\endgroup\$ – AlfroJang80 Jun 2 '19 at 2:20
  • \$\begingroup\$ In a low-side switch, the drain of the PMOS would be tied to Gnd. If you want the source to also be as close to ground as possible, then Vth means that the gate must be negative with respect to ground. Otherwise, you just have a source follower, and you waste a lot of power in the MOSFET. \$\endgroup\$ – Dave Tweed Jun 2 '19 at 2:22
  • \$\begingroup\$ @AlfroJang80 almost no current is conducted at vth. You need a lot more than vth to get a low Rdson, more than what source-drain voltage will drop to when the mosfet has a low Rdson. All the while, the sourve voltage gets closer to gnd as the Rdson decreases which means the gate voltage has to be even lower since gate-source voltage is all that the MOSFET can or does know and care about \$\endgroup\$ – DKNguyen Jun 2 '19 at 2:31

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